CN107076618B - Wavefront sensor and method for determining the presence of translational and tilting differences between several light beams - Google Patents
Wavefront sensor and method for determining the presence of translational and tilting differences between several light beams Download PDFInfo
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- G—PHYSICS
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- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
- G01J9/0215—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods by shearing interferometric methods
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J2009/002—Wavefront phase distribution
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Abstract
A wavefront analyser adapted to simply determine an initial wavefront (S)0) May be present in different regions of the optical system. To this end, the interference between the two waves is only caused by beams (F) from adjacent regions on the initial wavefront1、F2) And (4) generating. Such an analyzer may be used to coherently combine laser radiation produced by different sources arranged in parallel. Another application is for determining the difference in height and tilt that exists between adjacent mirror segments of a kakey telescope.
Description
The present invention relates to an interference based wavefront sensor. It also relates to a method for determining the translational (piston) and tilting differences existing between several beams capable of generating interference.
In this specification, the expressions "wavefront" and "wave front" are to be understood as synonyms. Similarly, the terms "interference pattern" and "interferogram" are also used as synonyms. The term "tilt" is used to denote the inclination of a wavefront, and "translational difference" denotes the average deviation of the travel existing between two wavefronts, each wavefront travel being measured in its direction of propagation.
The translational differences thus defined apply regardless of the spectral composition of the optical radiation. Therefore, it may also be referred to as an absolute translational difference. However, for monochromatic light radiation, only the residual part of the translational difference can be used during static characterization (i.e. characterization performed at a certain moment) due to the spatial periodicity of the electromagnetic field. This residual Part is called residual translation difference and is equal to [ Part _ Dec (Δ p/λ) ]. λ, where λ denotes the wavelength of the monochromatic radiation, Δ p is the absolute translation difference, and Part _ Dec denotes the fractional Part of the number contained between the brackets. In particular, when monochromatic light beams having the same wavelength are combined together, the residual translational difference existing between only two resultant light beams is significant. But, on the contrary, for pulses of optical radiation or for polychromatic optical radiation, only absolute translational differences are of interest.
Some applications require accurate determination of the translational differences and tilt that exist between the respective wavefronts of several beams.
This need arises particularly during mirror adjustment of a Keck telescope. The mirror is made up of juxtaposed individual mirror segments, each most often having hexagonal peripheral limits. It is thus possible to form a complete mirror of about ten meters in diameter, with sections of about one meter in diameter measured separately. However, the mirror segments have to be adjusted in height and inclination with respect to each other so that the slope of the wavefront of the light beam reflected by the complete mirror does not have steps or sudden changes due to differences in height and inclination existing between adjacent mirror segments.
This need also exists when beams from multiple laser sources are coherently combined to obtain the final high intensity beam. When the light intensity required in the composite beam is very high, the number of individual laser sources can be quite large. In the case of a monochromatic laser source, the individual wavefronts of the beams, which originate from the laser source respectively and correspond to the same phase value, have to be synthesized without phase errors. A paper entitled "La measure d' amplitudes complex phase multi-plate ral" [ measuring complex amplitude by multilateration ] which was answered by B-tulton (B Toulon) at university of Paris XI (university of Paris XI oray) in orls on 11, 20 th month 2009 specifically proposed a method based on quadrilateral shear interferometry for measuring translational differences and tilt between 64 laser sources. In the case of a pulsed laser source, the combination of independent pulses respectively generated by the laser source is not itself a pulse of similar duration to that of the independent pulses, unless there is no significant delay of some of the independent pulses relative to other pulses and there is no difference in their direction of propagation. For these applications of coherent synthesis of monochromatic light beams or light pulses, an interference-based wavefront sensor is used, comprising:
-a light input intended to receive optical radiation having an initial wavefront extending through the light input;
-a radiation separator arranged to produce, from the light beams respectively originating from the restricted areas in the light input, several sub-beams for each light beam, each sub-beam reproducing the characteristics of the initial wave front present in the corresponding restricted area;
-optical paths arranged to superimpose sub-beams originating from different confined areas within the light input and each passing via a different optical path;
-at least one image detector arranged to capture an interference pattern produced by the superimposed beamlets; and
-a processing module adapted to determine from the interference pattern a translation difference and a tilt existing in an initial wavefront between the confined areas from which the superimposed sub-beams originate.
A wavefront sensor is then used to characterize the overall wavefront from the individual wavefronts generated by the laser source alone.
In the device proposed by B-balun, the radiation separator is a diffraction grating that produces four replicas of the initial wavefront corresponding to a combination of two diffraction orders, each equal to +1 or-1. Thus, the radiation separator produces four sub-beams from each beam. The confined area within the light input corresponds to the portion of the individual beams originating from the juxtaposed laser sources. These are equipped with output microlenses such that the individual beams each have a parallel or collimated beam structure. The image detector then captures a combination of four-beam interferograms from which the translational differences and tilt that exist between two adjacent laser sources within the optical input can be determined. Two different interference modes are obtained depending on the orientation of the diffraction grating relative to the square distribution pattern of the laser source in the light input. In both modes, however, the interferogram has a complex structure with different classes of overlapping regions. Therefore, it is very difficult to determine the translation difference and the tilt from any one of the interferograms.
An article entitled "Collective phase measurement of an array of fiber lasers by four-wave lateral shear interferometry for coherent beam synthesis" published by berlange et al on pages 3931 to 3933 of volume 23 of the optical rapid report at 1/12/2010, volume 35, entitled "Collective phase measurement of an array of fiber lasers by four-wave lateral shear interferometry" relates to the same type of four-wave lateral shear interferometer.
Based on this situation it was an object of the present invention to allow a simpler determination of the translation differences and the tilt existing between the individual wavefronts of the beams capable of generating interference.
To this end, a first aspect of the invention proposes a wavefront sensor such as described above, but which also comprises a mask adapted to select, by means of openings in this mask, separate regions of interest within the light input as restricted regions, to at least partially block an initial wavefront outside these regions of interest, or to at least partially block light beams not originating from the regions of interest. Thus, if the mask is positioned close to the light input of the wavefront sensor, the region of interest may be determined directly by the opening in the mask, or the region of interest may be determined by optical conjugation via the sensor components. With such a mask, individual portions of the image detector are dedicated to adjacent pairs of regions of interest within the light input, respectively, regardless of the extension of the initial wavefront and the light intensity between two adjacent regions of interest. Each interferogram then only comprises an interference pattern with two beams within a portion of the image detector that can be separated from portions of other interferograms. Determining the translation and tilt differences from these interference patterns with two beams is relatively simple and can be implemented easily and quickly. In particular, the fourier transform of the interferogram need not be calculated.
In a preferred embodiment of the invention, the radiation separator may comprise a diffraction grating. In fact, when two sub-beams are generated by the diffraction grating for different diffraction orders, the photons in each sub-beam that contribute in the range of irradiation of the pulse are contained in a spatial section of propagation progress, said spatial section being parallel to the diffraction grating. For the same illumination pulse, the spatial sections of all sub-beams are aligned with each other, parallel to the diffraction grating. The overlap area where interference occurs between two of the sub-beams may be larger. The final accuracy may be greater for the values of translational and tilt differences inferred from the interference pattern.
More specifically, the wavefront sensor may be arranged such that the sub-beams produced by the diffraction grating for each beam correspond to the values of one or more diffraction orders +1 and-1. The mask then blocks all sub-beams originating from the region of interest, whose diffraction orders have a value of zero.
Advantageously, the mask and the image detector may be optically conjugated. In this case, the separated portions where the image detector generates interference correspond to the pair of openings of the mask. The images of the two openings of the same pair are superimposed on each other on the image detector by means of the radiation separator.
According to a refinement of the invention, the wavefront sensor may further comprise an afocal optical system arranged in the radiation path between the optical input and the image detector to transform the initial wavefront with respect to the interference pattern captured by the image detector by similar spatial scaling effects within the optical input. The selection of the magnification of the afocal system then enables the sensitivity to tilt differences between adjacent regions of interest to be adjusted independently with respect to the sensitivity to translation differences.
In a simple embodiment of the invention, the light input, the mask, the afocal optical system, the radiation separator, and the image detector may be arranged in this order according to the direction of propagation of the radiation within the wavefront sensor. In this case, the mask and the image detector may be optically conjugated by an afocal optical system through a radiation separator. This embodiment is particularly simple and combines all the advantages mentioned.
For example, the mask may be adapted to select the regions of interest based on a hexagonal network of these regions of interest within the distributed light input. Subsequently, the diffraction grating forming the radiation separator is a two-dimensional grating having a hexagonal pattern and is oriented such that the axis of symmetry of the diffraction grating is at 90 ° to the axis of symmetry of the mask with respect to the optical axis of the wavefront sensor. Such a hexagonal configuration fits the shape of the mirror section of the kaick telescope, as well as the compact arrangement of the fiber laser source.
Preferably, the mask is such that any two adjacent regions of interest have the same shape and the same size and are separated by a blocking region of the mask located between the two adjacent regions of interest, and this blocking region is large enough to contain a shape that is the same as the shape of each of the two adjacent regions of interest and the same size as the latter. Then, the surface portion of the detector formed with the interference pattern corresponding to the two regions of interest is surrounded by a zero-illumination circle. In other words, the portions of the surface of the detector occupied by the interferograms are separated from each other without overlap, which facilitates automatic detection and analysis of each interferogram in the entire image captured by the detector.
By means of the invention, the determination of the translation and tilt differences is relatively simple and can be carried out quickly and at low cost. For example, the processing module may include a library of stored reference patterns each made up of parallel straight-line interference fringes, each reference pattern being associated with a value of a translation difference and a value of a tilt difference. The values of the translation and tilt differences existing between two adjacent regions of interest are then deduced by searching for the maximum coincidence between the interference pattern corresponding to these two regions of interest and one of the stored reference patterns. Such searching for maximum coincidence may include applying illumination or light intensity scale corrections to the interference pattern and/or each reference pattern. Coincidence scores are then estimated and the estimated coincidence scores are compared to score values obtained for the same interference pattern, but the same interference pattern is compared to other reference patterns from the library.
In general, the processing module may be adapted to infer a value of a translation difference existing between two adjacent regions of interest from a lateral fringe displacement existing in an interference pattern corresponding to the two regions of interest. In addition, the value of the difference in tilt existing between the two regions of interest can be inferred from the inter-fringe spacing existing in the interference pattern.
Furthermore, the wavefront sensor according to the invention may further comprise a spectral separation system adapted to separate at least two spectral components of the light radiation received by the light input from each other. The wavefront sensor is then adapted to capture an interference pattern produced by the superimposed sub-beams separately for each spectral component and to determine a translation difference and a tilt difference for each spectral component from the interference pattern captured for this spectral component. For example, the spectral separation system may be of the spatial separation type for directing the spectral components to respective independent optical paths of the wavefront sensor. Alternatively, the spectral separation system may have a variable time shift according to wavelength, and interference patterns formed by different spectral components are captured at different times.
Finally, the mask and the radiation separator of the wavefront sensor according to the invention can be formed together by a spatial light modulator. Such an embodiment is advantageous because it is simple and can be adapted to the requirements. This is particularly suitable when the radiation separator comprises a diffraction grating.
A second aspect of the invention proposes to use a wavefront sensor according to the first aspect to determine the translational and tilting differences that exist between the individual wavefronts of the light beam that are capable of generating interference. To this end, the method of the invention comprises:
-providing an interference based wavefront sensor;
-directing each beam onto a different region of interest using adjacent regions of interest in the optical input of the wavefront sensor; and
-activating the image detector and the processing module to determine the translational and tilting differences existing between the independent wavefronts of the light beams directed onto adjacent regions of interest.
By means of the invention, the translation and tilt differences existing between the beams can be determined without using any additional reference waves. The method is therefore relatively simple to implement, without the need for optical components dedicated to the generation and introduction of such reference waves.
This method can be used to adjust the mirror segments of a kakey telescope. In this case, the beam is generated by a radiation source arranged such that radiation is reflected simultaneously by the juxtaposed sections of the mirror. For each section of the mirror, the portion of the radiation reflected by this section then forms a beam which is directed onto one of the regions of interest of the light input. For this application, the method also comprises calculating the difference in height and the difference in inclination existing between two adjacent segments of the mirror. These height and tilt differences are calculated from the values of the translation and tilt differences determined for the corresponding light beams.
The method according to the second aspect of the invention may also be used for adjusting the phase of the light beams respectively generated by separate laser sources, in particular fiber laser sources. When the laser source is of the pulsed laser type, the radiation separator advantageously comprises a diffraction grating. For this further application, the method further comprises calculating the time difference and the propagation direction difference existing between pulses of radiation generated by two different laser sources whose beams are directed onto two adjacent regions of interest. These time differences and propagation direction differences are calculated from the values of the translation differences and the tilt differences determined for the beams.
Further characteristics and advantages of the invention will become apparent from the following description of non-limiting embodiments thereof, with reference to the attached drawings, in which:
figure 1 is a schematic view in a single spatial dimension, illustrating the function of the mask proposed by the present invention;
figures 2a and 2b represent a mask and a corresponding image, respectively, produced by a wavefront sensor according to the present invention;
FIG. 3 is an optical diagram of a wavefront sensor for phase adjusting light beams generated by several laser sources according to the invention; and
fig. 4a and 4b are respective optical diagrams of two other wavefront sensors according to the invention for adjusting the mirror segments of a kaick telescope.
For the sake of clarity, the dimensions of the various elements shown in the figures do not correspond to actual dimensions or actual dimensional relationships. In addition, the same reference numerals given in different drawings refer to the same elements or those elements having the same functions.
The reference numerals used in fig. 1 have the following meanings:
optical axis of delta wave front sensor
10 wavefront sensor light input
11 mask with independent openings
12 diffraction grating
13 surface of image detector
14 processing module, labeled CPU
F0Initial light beam
F1、……、F4Initial beam F0In the selected beam
F’1、F”1Originating from the beam F1Is sub-beam
F’2、F”2Originating from the beam F2Is sub-beam
F’3、F”3Originating from the beam F3Is sub-beam
F’4、F”4Originating from the beam F2Is sub-beam
S0Initial wave front
ZI1、……、ZI4Region of interest
P12、P23、P34Part of the surface of an image detector
p initial wavefront S0Present in the region of (1)
i initial wavefront S0Is present in the region of
The light input section 10, the mask 11, the diffraction grating 12, and the image detector 13 are arranged perpendicular to the optical axis Δ. The mask 11 is arranged in the light input 10 or along the optical axis ΔArranged behind said light input 10. The openings in the mask 11, which are separated from one another, determine what are referred to as regions of interest in the light input 10 and are labeled ZI1、……、ZI4A plurality of regions of (a). First, an initial beam F may be assumed0Is monochromatic and propagates substantially parallel to the optical axis delta. Its initial wavefront S passing through the optical input 100Can have substantially any shape with translational changes parallel to the optical axis Δ and tilt changes relative to a plane perpendicular to the optical axis Δ. Thus, the opening of the mask 11 is only allowed to originate from the initial beam F0Is split beam F1、……、F4And passing through. Beam F1、……、F4Reproduction wave front S0Such as those respectively contained in the openings of the mask 11.
In the simple case of figure 1, with one dimension, the diffraction grating 12 derives the beam F from1、……、F4Generates two sub-beams which are symmetrically diffracted and correspond to the values +1 and-1, respectively, of the diffraction order. In fig. 1, the sub-beams hatched in one direction correspond to diffraction order +1, and those hatched in the other direction correspond to diffraction order-1. According to the invention, the surface of image detector 13 (hereinafter simply referred to as image detector 13) is positioned so as to receive, in the same portion thereof, both a sub-beam of diffraction order +1 and another sub-beam of diffraction order-1 (originating from the two beams determined by the adjacent openings of mask 11). Thus, the portion P of the image detector 1312Receiving the superimposed beamlets F "1And F'2. Similarly, part P of the detector23Receiving the superimposed beamlets F "2And F'3And part P34Receiving subbeam F'3And F'4. The mask 11 prevents the detector portion P12、P23And P34Receiving an initial beam F0Between the regions of interest ZI1With ZI2、ZI2With ZI3、ZI3With ZI4The middle portion therebetween. Diffraction orders greater than +1 or less than-1 can be ignored. In practice, by selecting an appropriate configuration of elements for the diffraction grating 12,the light intensity of these levels can be significantly reduced or cancelled. In each of the parts of the detector 13, the two sub-beams that are superimposed produce an interference pattern. By including in the area of interest ZI1、……、ZI4Initial wavefront S in one of0Is compared to a possibly offset and possibly inclined plane portion along the optical axis delta, then each detector segment P12、P23、P34The interference pattern in (1) is constituted by parallel fringes. Now, for a monochromatic initial beam F0And when the difference of tilt is zero for one of these interference patterns, the phase shift of the central fringe with respect to the optical axis of the beamlets is equal to Δ p · F/λ, where λ is the wavelength and Δ p is the initial wavefront S from which the two beamlets generating the interference in the detector part in question originate0Is present, and F is the inter-fringe spacing of the interference pattern. But in practice, due to the initial beam F0Only residual translational differences can be measured.
For in the detector part P12、P23、P34For the two sub-beams that produce the interference pattern, the difference in tilt Δ i is the sum of the initial wavefront S at the apex of the prism0The angle formed by two partially tangent planes contained in the region of interest in question. This difference of inclination Δ i is then combined with the difference of inclination of the beamlets forming interference in the section of the detector. This combination of angles is only simple if the direction of the apex of the prism of the difference in tilt is perpendicular to the plane containing the propagation directions of the two sub-beams. The combination is an addition of an angle and the variation of the spatial frequency of the interference fringes of the plane containing the propagation direction of the sub-beams (measured from the trajectory on the surface of the detector) is equal to the difference of tilt Δ i divided by the wavelength. In case of any orientation of the direction of the apex of the prism of the tilt difference with respect to the plane of the propagation directions of the two sub-beams, the person skilled in the art will use the mathematical formula given in the above-mentioned paper of B · coulomb. Thus, the tilt difference and their orientation difference can be deduced from the interference pattern of the two waves captured by the image detector. The interference pattern also generally provides redundancy, which can be used toThe accuracy of determining the orientation of the translation difference, the tilt difference, and the tilt difference is improved.
Fig. 2a and 2b illustrate a two-dimensional implementation of the operation of the invention just described. FIG. 2a is a graph having a structural formula with Z1、……、Z4A front view of the mask 11 of the different regions of interest, generally designated ZI. The area of interest ZI is thus determined by the openings arranged in the opaque plates forming the mask 11. When the mask 11 is positioned in the light input 10, the region of interest merges with the opening of the mask. Two adjacent regions of interest ZI are separated from each other by an opaque middle section of the mask. The width of this intermediate section is at least equal to the size of each of the two adjacent regions of interest. Preferably, the regions of interest ZI are distributed in a conventional hexagonal network, wherein a11Is one of the axes of symmetry or the principal axis of alignment of the region of interest of the mask 11 with sixth order symmetry. The areas of interest ZI may be circular, wherein the area size is sufficient to ignore light diffraction produced by these areas of interest. For example, the regions of interest ZI may each have a diameter of 50 μm (micrometers), and the geometric centers of two adjacent regions of interest may be 110 μm apart. In this case, the diffraction grating 12 is also a conventional hexagon, but wherein one of its symmetry axes A is12On two axes A11And A12Perpendicular to axis A when projected in the same plane perpendicular to optical axis Delta11。
Fig. 2b shows a complete image captured by the detector 13 for the mask 11 in fig. 2a, wherein parts of the surface of the detector are individually dedicated to adjacent pairs of regions of interest ZI. Except for the part P presented separately with reference to FIG. 112、P23And P34Except that these parts of the surface of the detector 13 are generally denoted as P. For some of the portions P, the interference pattern formed by the parallel fringes is also shown by illustration. Of course, similar interference patterns exist in all portions P. Unwanted interference with more than two waves and an initial wavefront S within each region of interest ZI can be ignored0Is changed.
The analysis of the image captured by the detector 13 can be started by automatically detecting the portion P containing the interferogram. This automatic detection is facilitated by the fact that the portions P are independent, i.e. wherein there is no overlap between adjacent portions and the intermediate section does not receive any light flux between two adjacent portions P. The automatic detection of the portion P is also facilitated by a priori knowledge of the position and shape of the portion P on the surface of the detector 13.
The independent processing of each interferogram can be implemented in many ways to obtain the values of the translational and tilt differences. A method that is particularly fast and does not require a large amount of processing power includes: each interferogram contained in the portion P is compared with the originally stored interferogram having two plane waves and known values of translational and tilt differences. This method therefore works by comparing the content of the images and is very effective for images formed by parallel stripes. This approach is also more robust with respect to artifacts that may be present in the image, such as effects of unwanted interference with more than two waves, tilt variations within each region of interest, and unwanted diffraction. In a known manner, image comparison methods generally comprise an initial step during which, for the two images compared, the mean and standard deviation of the brightness of the images are set to the same value. To this end, an affine scaling may be applied to the luminance values of at least one of the two images compared. The coincidence score of the two images is then calculated. A library of interference patterns with two waves is then initially provided, indexed by the values of the translation difference and the tilt difference, and also indexed by the angular orientation value of the tilt difference. In the general description of the invention, such interference patterns stored in advance have been referred to as reference patterns. For each interferogram of the complete image captured by the detector 13, the values of the translation and tilt differences and optionally the angular orientations of the tilt differences are thus those containing the reference patterns in the library with the highest coincidence scores. Alternatively, when variable rotation is applied to the interferograms or reference patterns, the angular orientation of the tilt difference can be inferred from a comparison of each interferogram to each reference pattern.
Fig. 3 illustrates the use of a wavefront sensor according to the present invention to achieve coherent combining of light beams generated by a set of fiber laser sources 1000. The laser sources 1000 can cause interference with each other and are flatArranged in rows such that the output sections of the optical fibers are all substantially arranged in the same plane perpendicular to the optical axis Δ. Each fiber is equipped with an output lens to collimate the light beam originating from that fiber. The number of optical fibers that generate the individual laser beams is thus unlimited and may, for example, be of the order of hundreds of thousands. All individual beams of the laser source 1000 enter the light input 10 substantially parallel to the optical axis Δ. The mask 11 has at least as many openings as the number of laser sources 1000. The individual fibers are thus guided into the openings for which the mask 11 is dedicated, and it is advantageous to use as many adjacent, grouped openings as possible. The lateral distribution of the optical fibres may be in a hexagonal network so that the mask 11 and grating 12 described with reference to figures 2a and 2b may be used. The laser beams originating from the optical fibers thus correspond respectively to the previously introduced beams F1、……、F4… …. Reference numerals 101 and 102 denote two converging lenses, wherein their focal lengths are respectively designated by f1And f2. Which are arranged together to form an afocal optical system indicated by reference numeral 100. In other words, the image focus of lens 101 is superimposed on the principal focus of lens 102. The two lenses 101 and 102 have sufficient lateral extension to contain all the beams originating from the optical fiber. In addition, the surface of the image detector 13 is placed along the optical axis Δ to be optically conjugate with the mask 11 through the two lenses 101 and 102. For example, the mask 11 may be located at the level of the main focus of the lens 101 and the surface of the image detector 13 may be located at the level of the image focus of the lens 102. The diffraction grating 12 may be inserted between the lens 102 and the surface of the image detector 13. The exact position thereof along the optical axis Δ is adjusted such that the portions P of the detector 13 are each a superposition of the images of two adjacent openings of the mask 11. In such a configuration of the wavefront sensor, the portions P form a hexagonal network having voids in one-to-one correspondence with locations where the image of the openings in the mask 11 would be located in the absence of the grating 12. Fig. 2a and 2b show such a correspondence. Thus, such embodiments of a wavefront sensor according to the present invention enable determination of the translation and tilt differences existing between adjacent optical fibers when all fiber laser sources 1000 are monochromatic with the same common wavelength.
In addition, the magnification of the afocal optical system 100 enables the sensitivity of the wavefront sensor to be adjusted for tilt differences without changing its sensitivity to translational differences. This change in sensitivity to tilt difference is obtained from the Gouy theorem. Selecting a low value for the magnification of the afocal optical system 100 (specifically a magnification value less than one) enables a wavefront sensor more suitable for accurately measuring tilt differences to be obtained.
The embodiment of fig. 3 is specific to the laser source 1000 being of the pulse type, each laser source being adapted to deliver very short pulses of radiation, e.g., on the order of picoseconds or less. It is assumed that the possible tilt differences between pulses originating from different laser sources have been compensated or corrected elsewhere. Due to the following facts: for embodiments using a diffraction grating, the inter-fringe spacing is independent of wavelength, so each interference pattern within one of the portions P of the detector 13 is still formed by fringes separated according to a defined inter-fringe spacing. For each wavelength analyzed, the residual translational difference can be inferred from the lateral displacement of the middle fringe of the corresponding interference pattern. However, the aim of this application of the invention to a pulse modulation regime is in fact to find the absolute translation differences existing between pulses originating from different sources, measured in the direction of co-propagation of the pulses. For two pulses originating from adjacent sources, the absolute translational difference existing between the latter is equal to the residual translational difference determined for each wavelength used for detection plus an integer multiple of this detection wavelength. This uncertainty can be resolved by measuring the translation difference simultaneously for at least two different wavelengths. The absolute translational difference between two pulses originating from adjacent laser sources can thus be determined, the closer the wavelengths used to form the interference pattern, the greater the absolute translational difference. The use of two spectral spacings that are very narrow around two different wavelengths is sufficient in most cases.
Such measurements at multiple wavelengths may be obtained by appropriate spectral filtering to select components of the radiation corresponding to different spectral intervals and by directing each filtered component of the radiation to a separate path of the wavefront sensor. An alternative method may include spreading each pulse over an extended duration by forming a time shift that varies according to the frequency of the spectral components that make up the pulse. Such methods of spectrum-time (spectro-temporal) extension are known to those skilled in the art. Wavefront analysis according to the present invention can be performed at different wavelengths when implemented at different times over the extended duration of the extended pulse. To this end, multiple independent wavefront sensor paths may still be provided in parallel and activated at different times.
Fig. 4a and 4b illustrate another application of a wavefront sensor according to the present invention for measuring the difference in height and tilt that may exist between adjacent sections of a mirror of a kayak telescope. Based on the differences to be measured in this way, the relative positions of the segments of the mirror can be readjusted so that the wavefront resulting from the reflection across the mirror has no abrupt changes in steps or slopes.
In fig. 4a, reference numerals 101 and 102 also denote two converging lenses forming a first afocal optical system. At the same time, lens 101 forms with converging illumination lens 103 a second afocal optical system, the focal length of which is denoted f3. The beam splitter 104 enables coupling of the illumination path and the output path to the same optical test path. The illumination path includes a laser source 2100 and a lens 103. Light beam F produced by source 21000Is directed into the optical test path by the beam splitter 104. The optical test path comprises a lens 101, a mask 11, a diverging lens 2200 and a test mirror, which is denoted by reference numeral 2000. The mirror 2000 is composed of all juxtaposed mirror segments 2001, 2002, 2003, etc. These mirror segments are juxtaposed in a hexagonal network to use masks and diffraction gratings such as those described above. The diverging lens 2200 is selected and positioned relative to the mirror 2000 to produce, in combination therewith, an optical function equivalent to a flat mirror, without regard to any imperfections in the relative positions of the individual sections 2001, 2002, 2003, etc. The purpose of the present application of the invention includes determining these defects in the relative positions. The diverging lens 2200 may be partially cylindrical according to the mirror 2000. The output path includes the lens 102, the diffraction grating 12 and the image detector 13.
In the wavefront sensor of fig. 4a, the mask 11 is located between the lens 101 and the lens 2200. The mask is designed to have light beams F selected to originate from the respective primary mirror regions 2001, 2002, 2003, etc1、F2Etc. and blocks the light beam F0Portions of the separation gap existing between adjacent sections of the mirror 2000 will be illuminated. In fact, to implement such a wavefront sensor, for the beam F0The portion reflected by all segments of the mirror and propagating from right to left in fig. 4a may be considered to have the light input 10 at the level of the mask 11. The optical test path and the output path together constitute a wavefront sensor assembly similar to that shown in figure 3.
Measuring the inter-fringe spacing of the interferograms contained in each portion P of the detector 13 provides a corresponding beam F1、F2Etc., and then the difference in tilt existing between the corresponding mirror segments 2001, 2002, etc., is provided by the adjacent pair of mirror segments. When source 2100 is monochromatic, the position of the central fringe of each interferogram enables the residual translational differences existing between the beams reflected by two adjacent mirror segments to be determined. The use of at least two different wavelengths also enables the absolute translational difference to be taken and then the height difference existing between all segments of the mirror 2000 to be determined by the pair of adjacent mirror segments.
The diagram in fig. 4b is a variant of the application in fig. 4a for obtaining an application of the invention that can be implemented at the installation site of a kakey telescope. Reference numeral 2000 denotes a primary mirror of the kakey telescope having mirror sections 2001, 2002, 2003, etc. Reference numeral 3000 denotes a sub-mirror of the telescope, for example, in the case where the telescope has two mirrors. The lens 105 has a collimating function and forms the light input of the wavefront sensor. In this variant of the wavefront sensor, the mask 11 and the diffraction grating 12 may be side by side and optically conjugated to the image detector 13 by the afocal system 100. They are also optically conjugated to the mirror 2000. The radiation used for characterizing the difference in height and the difference in inclination of the mirror segments 2001, 2002, 2003, etc. can then originate directly from the star EAnd reaches the light beam F of the main mirror 20000. The mask 11 is also designed to block the beam F0Reaching the part of the main mirror 2000 in the horizontal plane in the intermediate gap between adjacent mirror segments and the part in the peripheral part of the mirror segments.
It will be appreciated that the present invention may be reproduced while modifying many details of implementation relative to the above description and yet retain at least some of the above advantages. Among the possible modifications, the following are non-limiting:
the radiation separator may be constituted by a mirror instead of a diffraction grating;
the distribution network of the regions of interest defined by the mask may be square or other, instead of hexagonal. The pattern of the diffraction grating can be adjusted accordingly;
the order of the optical components constituting the wavefront sensor can be modified by optical equivalence. In particular, the mask, grating and afocal optical system may be arranged in a different order while following the direction of propagation of the radiation within the wavefront sensor;
the afocal optical system can have a different structure from that already described with two converging lenses;
by the depth of field effect, the mask can be shifted to a greater extent along the optical axis of the wavefront sensor while still maintaining almost the same effect in the captured image; and
the wavefront sensor according to the invention can be used in many applications in addition to those already described.
Claims (14)
1. An interference-based wavefront sensor comprising:
-a light input (10) intended to receive an initial wavefront (S) having a wave front extending through said light input0) Light radiation of (2);
-a radiation separator arranged to separate light beams (F) originating from confined areas within the light input (10) respectively1、F2) To generate at least two sub-beams (F ') for each beam'1、F”1、F’2、F”2) Each beamlet reconstruction resides inThe initial wavefront (S) in the corresponding restricted area0) The characteristics of (a);
-an optical path arranged to superimpose two sub-beams (F) originating from two different confined areas within the light input (10) respectively and passing via different optical paths respectively "1、F’2)；
-at least one image detector (13) arranged to capture a beamlet (F) superimposed by the image data "1、F’2) The resulting interference pattern;
-a processing module (14) adapted to determine from the interference pattern the initial wavefront (S) between the confined areas from which the superimposed sub-beams originate0) The presence of a translation (p) difference and a tilt (i) difference; and
-a mask (11), the mask (11) being adapted to select, by means of an opening in the mask, a separate region of interest (ZI) within the light input (10) as a restricted region by at least partially blocking an initial wavefront (S) outside the region of interest0) Or by at least partially blocking light beams not originating from the region of interest, such that separate portions (P) of the image detector (13) are dedicated to adjacent pairs of regions of interest within the light input, respectively,
the wavefront sensor is characterized in that the mask (11) and the radiation separator are formed together by a spatial light modulator.
2. The wavefront sensor of claim 1, wherein the radiation separator comprises a diffraction grating (12).
3. The wavefront sensor as claimed in claim 2, arranged such that for each light beam (F) by the diffraction grating (12)1、F2) The sub-beam (F ') produced'1、F”1、F’2、F”2) Values +1 and-1 corresponding to one or more diffraction orders.
4. The wavefront sensor according to claim 1, wherein the mask (11) and the image detector (13) are optically conjugated.
5. The wavefront sensor of claim 1, further comprising an afocal optical system (100), the afocal optical system (100) being arranged on a radiation path between the optical input (10) and the image detector (13) to transform the initial wavefront (S) with respect to the interference pattern captured by the image detector by similar spatial effective scaling within the optical input (S)0)。
6. The wavefront sensor according to claim 2, wherein the mask (11) is adapted to select the region of interest (ZI) according to a hexagonal distribution network of the region of interest within the optical input (10), and the diffraction grating (12) is a two-dimensional grating having a hexagonal pattern and is oriented such that a symmetry axis of the diffraction grating and an alignment main axis of the region of interest of the mask (11) having a sixth order symmetry are in a plane perpendicular to an optical axis (Δ) of the wavefront sensor 90 °.
7. The wavefront sensor according to claim 1, wherein the mask (11) is such that any two adjacent regions of interest (ZI) have one and the same shape and one and the same size and are separated by a blocking region of the mask located between the two adjacent regions of interest, the blocking region being large enough to contain a shape identical to the shape of each of the two adjacent regions of interest and identical to the size of the two adjacent regions of interest.
8. The wavefront sensor of claim 1, wherein the processing module (14) comprises a library of stored reference patterns each constituted by parallel straight-line interference fringes, each reference pattern being associated with a value of the translation difference and a value of the tilt difference,
and wherein the values of the translation and tilt differences existing between two adjacent regions of interest (ZI) are deduced by searching for the maximum coincidence between the interference pattern corresponding to the two regions of interest and one of the stored reference patterns.
9. The wavefront sensor of claim 1, wherein the processing module (14) is adapted to
Inferring the value of the translation (p) difference existing between two adjacent regions of interest (ZI) from the lateral fringe displacements existing in the interference pattern corresponding to said two adjacent regions of interest (ZI), an
Inferring the value of the tilt (i) difference existing between two adjacent regions of interest (ZI) from the inter-fringe spacing existing in the interference pattern corresponding to said two adjacent regions of interest (ZI).
10. The wavefront sensor of claim 1, further comprising:
-a spectral separation system adapted to separate at least two spectral components of the optical radiation received by the optical input (10) from each other,
and the wavefront sensor is adapted to capture individually for each spectral component a beamlet (F) superimposed by the spectral component "1、F’2) The generated interference pattern and adapted to determine a translation (p) difference and a tilt (i) difference for each spectral component from the interference pattern captured for said spectral component.
11. For determining a light beam (F) capable of generating interference1、F2) Is characterized by comprising the following steps:
-providing an interference based wavefront sensor according to any of claims 1 to 10;
-using adjacent regions of interest in the light input (10) of the wavefront sensor to couple each light beam (F)1、F2) Directing onto different areas of interest (ZI); and
-activating the image detector (13) and the processing module (14) to determine a light beam (F)1、F2) Independent waves directed onto adjacent regions of interest (ZI)The difference in translation (p) and the difference in tilt (i) existing between the fronts.
12. Method according to claim 11, wherein said light beam (F)1、F2) Is generated by a radiation source (2100), the radiation source (2100) being arranged such that the radiation is reflected simultaneously by juxtaposed sections (2001, 2002) of a mirror (2000),
and for each segment (2001, 2002) of the mirror, the portion of the radiation reflected by the segment of the mirror forms the beam (F)1、F2) Said light beam (F)1、F2) Is directed onto one of the regions of interest (ZI) of the light input (10),
and the method further comprises calculating a height difference and a tilt difference existing between two adjacent segments (2001, 2002) from the values of the translation (p) difference and the tilt (i) difference determined for the corresponding light beams.
13. Method according to claim 11, wherein said light beam (F)1、F2) Are generated by separate laser sources (1000), in particular fiber laser sources.
14. The method according to claim 13, wherein the laser source (1000) is of the pulsed laser type and the wavefront sensor is according to claim 2,
and the method further comprises calculating, from the values of the translation (p) difference and the tilt (i) difference determined for the beam, the time difference and the propagation direction difference existing between the radiation pulses generated by two different laser sources, the beam (F) of which1、F2) Are directed onto two adjacent regions of interest (ZI).
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| FR1458876A FR3026181B1 (en) | 2014-09-19 | 2014-09-19 | WAVE SURFACE ANALYZER AND METHOD FOR DETERMINING EXISTING PISTON AND TILT VARIANCES BETWEEN MULTIPLE LUMINOUS BEAMS |
| FR1458876 | 2014-09-19 | ||
| PCT/EP2015/071519 WO2016042161A1 (en) | 2014-09-19 | 2015-09-18 | Wavefront analyser and a method for determining differences in amplitude and tilt existing between several light beams |
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| CN107076618A CN107076618A (en) | 2017-08-18 |
| CN107076618B true CN107076618B (en) | 2019-12-27 |
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| EP (1) | EP3194916B1 (en) |
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| CN112130337B (en) * | 2020-10-14 | 2022-05-13 | 中国人民解放军国防科技大学 | Synchronous control system and method for phase and inclined phase of fiber laser array piston |
| FR3143741A1 (en) | 2022-12-20 | 2024-06-21 | Office National D'etudes Et De Recherches Aérospatiales | OPTICAL COMPONENT AND WAVE SURFACE ANALYZER COMPRISING SUCH OPTICAL COMPONENT |
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| JP2005345260A (en) * | 2004-06-03 | 2005-12-15 | Canon Inc | Flat surface interference measuring method and flat surface interference measuring device |
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| US5080490A (en) * | 1990-05-23 | 1992-01-14 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Equal path, phase shifting, sample point interferometer for monitoring the configuration of surfaces |
| JP2536359Y2 (en) * | 1991-04-30 | 1997-05-21 | アルプス電気株式会社 | Optical writing head |
| US5220406A (en) * | 1991-09-27 | 1993-06-15 | Hughes Danbury Optical Systems, Inc. | Sample point interferometer having separate rigid body and figure measurement channels |
| FR2712978B1 (en) * | 1993-11-24 | 1996-02-02 | Onera (Off Nat Aerospatiale) | Achromatic optical interferometer, of the trilateral shift type. |
| US7106457B1 (en) * | 2003-05-29 | 2006-09-12 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Achromatic shearing phase sensor for generating images indicative of measure(s) of alignment between segments of a segmented telescope's mirrors |
| FR2897426B1 (en) * | 2006-02-16 | 2012-07-27 | Onera (Off Nat Aerospatiale) | WAVE SURFACE ANALYSIS METHOD BY MULTILATERAL INTERFEROMETRY WITH FREQUENCY DIFFERENCE |
| FR2930336B1 (en) | 2008-04-22 | 2010-05-14 | Onera (Off Nat Aerospatiale) | METHOD, PHASE NETWORK AND WAVE SURFACE ANALYSIS DEVICE OF A LIGHT BEAM |
| WO2010077675A2 (en) * | 2008-12-09 | 2010-07-08 | Zygo Corporation | Two grating lateral shearing wavefront sensor |
| JP2013050441A (en) * | 2011-08-03 | 2013-03-14 | Canon Inc | Wavefront measuring apparatus, wavefront measuring method, program and x-ray imaging apparatus |
| CN202420688U (en) * | 2011-12-20 | 2012-09-05 | 四川大学 | Hartmann wavefront sensor based on diffraction grating arrays |
| CN103033272B (en) * | 2012-12-21 | 2014-11-19 | 南京信息工程大学 | Simultaneous phase-shifting lateral shearing interferometer and detection method |
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